1. Field of the Invention
The present invention relates to the field of data transmission in communications networks comprising one or more low-bit-rate arteries or comprising arteries with low-bit-rate channels.
The invention can be applied especially in the field of telecommunications and relates more particularly to a method to implement the AAL2 (ATM Adaptation Layer 2) protocols for conveying data within an ATM (asynchronous transfer mode) switching network comprising one or more low-bit-rate arteries.
It is used for example in networks comprising lines using satellite links, radio or any other type of links where the bandwidth resources may be highly limited.
It can be applied for example in all networks providing different services such as telephony, voice communications, fax, and data transmission by modem.
ATM networks have been designed for the high-bit-rate conveying of multiservice data (voice, video, data, etc.). They switch fixed-length cells comprising 48-byte payloads.
2. Description of the Prior Art
To this end, in the earliest implementation of ATM telephone services, it was sought to recreate a service comparable to that provided by synchronous public switching networks. The voice is digitized at 64 kbits/s. The ATM cells are filled sequentially with 47 speech samples and with one additional byte giving a sequence number that authorizes the detection of cell losses, thus enabling the generation of replacement noise for lost information. This implementation is known as the AAL1 (ATM Adaptation Layer 1) standardized by the ITU in its recommendation [1.363.1] of 1996 (ITU specification for the AAL type 2 ISDN-LB ATM adaptation layer).
Subsequently, in 1997, the ITU standardized a new adaptation layer (AAL2) dedicated to the transmission, in an ATM network, of variably sized application packets at low bit rates in its recommendation [I.363.2] (ITU specification for the AAL type 2 ISDN-LB ATM adaptation layer). This new protocol is more specifically designed for the real-time services which make use of efficient compression techniques. Video compression was an application commonly envisaged for this adaptation layer since the bit rates needed for full-band video meant that it was not possible to envisage any commercially viable operation. Another useful application of the adaptation layer pertains to the creation of trunking between small numbers of local telephone networks or private automatic branch exchanges (PABX)
Compression techniques produce packets of variable length with a relatively low bit rate. The AAL2 adaptation layer is used to convey these packets in ATM cells while offering the possibility that several packets will be in the same cell or that the packets will be split into two parts conveyed in consecutive cells at the time of transmission.
In certain fields of application, subject to operational constraints, the bandwidth used may be reduced and the ATM network arteries have a low bit rate, for example 2 Mbit/s, as compared with the usual bit rates which are in the range of 50, 155 or 622 Mbits/s. Furthermore, considering the requirements of connectivity between remote regions and a metropolitan area call for low-bit-rate links, for example 64 kbits/s, especially by satellite, voice transmission still takes up a major proportion of the transmission in these networks, and it has been necessary to use voice encoding techniques that are more economical in terms of bandwidth than the civilian PCM (pulse code modulation) technique according to the [G.711] recommendation working at 64 kbits/s, as is the case for example with the 16 bits/s Delta encoding system.
At the same time, the development of requirements in the field of civilian communications has led to the emergence of even more efficient techniques of voice encoding and compression: this is especially the case with the compression techniques implemented in GSM mobile telephony networks, or again with the technique of encoding at 5.3 kbits/s specified in the ITU [G.723.1] recommendation (this is a dual rate speech coder standard for multimedia communications transmitting at 5.3 and 6.3 kbit/s) and used in IP (Internet Protocol) voice communications.
In telephone communications compressed by a [G.723.1] type algorithm in a network where certain arteries are low-bit-rate arteries, the initial data, which have a certain length, take the form of small-sized frames after compression. For example, the frames that come out of a vocoder are small-sized frames (typically, 1, 4 or 20 bytes) but correspond to a duration of speech that is long (typically: 30 ms). To use the bandwidth with the utmost efficiency, it is possible to consider placing several of these frames in an ATM cell, but a great deal of time is needed to fill the cell before it can be sent. This will create a high latency time that may impair the subjective quality of the communications or may result in inconvenient echo phenomenal.
One way to get around this obstacle would be to multiplex several distinct calls in one and the same ATM virtual circuit. For example, if two calls are simultaneously active, it is possible to place a frame of one call and a frame of the other call in each ATM cell. This allows the cell to be sent within half the time that is needed when only one call is active. However, this possibility of multiplexing requires that both calls should be made between the same pair of access points of the ATM network. This is not the most usual situation.
In general, the idea of the invention consists in optimizing the transfer of data in a communication network comprising one or more low-bit-rate arteries, wherein the data, when compressed, takes the form of frames whose size is smaller than the size of the basic transmission unit, for example a cell, and the network implements an adaptation layer type of protocol to distribute and multiplex the small-sized frames in a fixed-size packet in order to route them in the network.
The Idea of the Invention consists, for example, in shaping certain data, especially upstream to the low-bit-rate arteries or at the entry to lines comprising one or more low-bit-rate channels and in conveying the data in a network of this kind.
In this description, the expression “low-bit-rate arteries” designates either a low-bit-rate arteries or an arteries comprising one or more low-bit-rate channels.
The communications network is characterized especially by means of a basic transmission unit, for example a cell or a fixed-size container such as an ATM cell or an adaptation protocol or an adaptation layer protocol.
When applied to the AAL2 protocol, the invention relates to a specific implementation of this AAL2 protocol which optimizes the padding of the cells on the low-bit-rate channels without, however, introducing any excessive delay in telephone communications.
The invention also comprises a component related to the search for a route passing through the units capable of carrying out this implementation of the AAL2 protocol and a component related to the detection of information losses in order to manage a piece of conventional substitute information in order to maintain binary integrity or synchronism.
The terms “upstream” and “downstream” are considered with respect to a direction of circulation of data in the network.